To an easterner like me, the signs of autumn in San Francisco are few and far between -- the homeopathic scent of decayed oak leaves on certain rainy street corners, say, or the decreasing span of the day as the sun dips ever more sharply over the Pacific rim. I happened to watch the sunset last week from the towering observation deck of Jacques Herzog and Pierre de Meuron’s splendid new de Young Museum. It sits in Golden Gate Park like a contemporary castle keep, a sprawling palace of art clad in stippled bronze, fit for a king’s collection -- and built at a king’s ransom, some $135 million.